Drew Barrymore tries so hard in “Grey Gardens.” She earns an A for effort — and a D for results.

All her straining undermines the HBO movie, which debuts at 7 p.m. Saturday on Sunflower Broadband Channels 301 and 401.

“Grey Gardens,” the true saga of eccentric “Big Edie” Bouvier Beale and daughter “Little Edie,” was first a documentary and later a Broadway musical. The dramatic version unfolds like a lush soap opera that goes horribly wrong.

Grasping Big Edie and dreamy Little Edie could have wandered out of a Tennessee Williams play. Big-spending Big Edie (Jessica Lange) exasperates her husband (Ken Howard), who leaves.

Big Edie lets her house fall apart and her cats run wild. But she won’t let go of sensitive Little Edie (Barrymore), whose hair falls out from stress. The movie bounces between the 1970s, when the documentary recorded their bleak home life, and earlier, plusher times in their lives.

Barrymore looks lovely as young Little Edie, but she adopts a wobbly accent that grates. She sounds like a bad Katharine Hepburn impersonator. The actress fumbles as Little Edie ages 40 years; Barrymore looks ready for Halloween rather than this movie.

Lange, however, is brilliant. She plays deluded forcefully, ages credibly and brings pathos to pathetic behavior.

The Beales hold fascination because they were related to Jacqueline Kennedy Onassis (Jeanne Tripplehorn of “Big Love”). Onassis visits briefly to suffer Little Edie’s taunts and to tell her relatives that the glamorous life isn’t all it’s cracked up to be.

Onassis has been the subject of many awful TV movies. “Grey Gardens” is thoughtful and swank but ultimately mediocre. Two actresses in top form are required to put this story across.